Enter the portal to begin the final instance of the Clockwork Chaos story. You can complete this instance by yourself, or with a full group. Only one person in the party needs to have completed the portal device in order to enter.
Quote is from official Anet guide
The only one person in the party needs to have the portal device only applies if everyone is on the same server. So everyone needs to guest to the same server to start or be able to open it themselves to join the instance.